<p>Total parenteral nutrition General Usage TPN is normally used following surgery, when feeding by mouth or using the gut is not possible, when a person's digestive system cannot absorb nutrients due to chronic disease, or, alternatively, if a person's nutrient requirement cannot be met by enteral feeding (tube feeding) and supplementation. It has been used for comatose patients, although enteral feeding is usually preferable, and less prone to complications. Short- term TPN may be used if a person's digestive system has shut down (for instance by Peritonitis), and they are at a low enough weight to cause concerns about nutrition during an extended hospital stay. Long-term TPN is occasionally used to treat people suffering the extended consequences of an accident or surgery. Most controversially, TPN has extended the life of a small number of children born with nonexistent or severely birth-deformed guts. The oldest were eight years old in 2003. The preferred method of delivering TPN is with a medical infusion pump. A sterile bag of nutrient solution, between 500 mL and 4 L is provided. The pump infuses a small amount (0.1 to 10 mL/hr) continuously in order to keep the vein open. Feeding schedules vary, but one common regimen ramps up the nutrition over a few hours, levels off the rate for a few hours, and then ramps it down over a few more hours, in order to simulate a normal set of meal times. The nutrient solution consists of water, glucose, salts, amino acids, vitamins and (more controversially) sometimes emulsified fats. Long term TPN patients sometimes suffer from lack of trace nutrients or electrolyte imbalances. Because increased blood sugar commonly occurs with TPN, insulin may also be added to the infusion. Occasionally, other drugs are added as well. Chronic TPN is performed through a Hickman line or a Port-a-Cath (venous access systems). In infants, sometimes the umbilical artery is used. Battery-powered ambulatory infusion pumps are used with chronic TPN patients, and usually the pump and a small (100 ml) bag of nutrient to keep the vein open are carried in a fanny pack. Outpatient TPN practices are still being refined. Aside from their dependence on a pump, chronic TPN patients live quite normal lives. Complications The most common complication of TPN use is bacterial infection, usually due to the increased infection risk from having an indwelling central venous catheter. Liver failure may sometimes occur; a recent study at Children's Hospital Boston on the cause suggests it is due to a large difference in omega-6 to omega-3 ratio. When treated with a different fatty acid infusion (which is not approved for use in the U.S.) two patients were able to recover from their condition.</p>